Finding change in internal energy

[BOAS]

Homework Statement

In exercising, a weight lifter loses 0.150 kg of water through evaporation, the heat
required to evaporate the water coming from the weight lifter's body. The work done
in lifting weights is 1.40x10⁵ J. (a) Assuming that the latent heat of vapourisation
of perspiration is 2.42x10⁶ Jkg^-1, find the change in the internal energy of the weight lifter.

Homework Equations

Q = mL

Δu = Q - w

The Attempt at a Solution

Q = mL
= 0.15 * 2.42x10⁶
= 363000 J

This is the amount of energy required to vapourise the perspiration.

Δu = Q - w

= 363000 - 1.40x10⁵

= 223000 J

Is this correct?

I was half expecting my answer to be negative since the weight lifter will have less energy after lifting weights, but I suppose it makes sense that he would also get hot which is what internal energy measures.

ETA The next part of the question goes on to say this;

(b) Determine the minimum number of nutritional calories of food (1 nutritional
Calorie = 4186 J) that must be consumed to replace the loss of internal energy.

Which supports my feeling that I'm wrong...

One thought I'm having is that since the energy required to vapourise the perspiration is coming from the body, I should be giving it as a negative figure.

 

[Chestermiller]

Q in the first law expression is defined as the heat gained by the body. In your example, Q should be negative because evaporation of sweat causes the body to lose heat.

 

[BOAS]

I thought that would be the case, thanks.